Thermometers which measure temperature by means of heat sensitive materials which undergo a change of state at a defined temperature are well known in the art. These thermometers may be either disposable, that is, the change of state may be irreversible, (as for example, in the prior art thermometers wherein the heat sensitive materials melt and contact a dye which is soluble therein) or non-disposable, that is reversible. Examples of thermometers which utilize heat sensitive materials that undergo a change in state from an opaque solid to a transparent liquid at a predetermined temperature may be found in U.S. Pat. No. 3,946,612, 3,175,401, 3,465,590 and 3,665,770.
In U.S. Pat. No. 3,046,612 the heat sensitive materials may be a solid solution, e.g. a mixture of o-chloro and o-bromonitrobenzene or various other mixtures of organic compounds. In this reference, it is noted that by incorporation of a third component to depress the melting point, higher melting solid solutions can be used for applications wherein it is desirable to measure lower temperatures. It is nowhere taught, shown nor suggested in this reference that said third component could be a polymer.
In U.S. Pat. No. 3,175,401 a reversible thermometer is disclosed which utilizes mixtures of dichlorobenzene and diphenyl or various other organic compounds as the heat sensitive material. To obtain reversibility as well as ease of reading, brightly colored glass chips are suspended in the heat sensitive material which, upon the melting thereof, sink to the transparent bottom of the compartment holding said heat sensitive material. To reuse this thermometer, the heat sensitive material must be remelted and the thermometer inverted. It is noted that this is a time consuming step, but illustrates one solution to the problem of providing a reversible, easily readable thermometer.
U.S. Pat. No. 3,465,590 uses fatty acids, including the binary and ternary mixtures thereof as the heat sensitive material. There is no teaching of using fatty acids in admixtures with polymers.
U.S. Pat. No. 3,665,770 describes the known problems with change of state thermometers. One problem of particular interest is that the change of state is not maintained for very long unless made irreversible. More particularly, the patentee points out that when the thermometer is removed from a patient's mouth it may quickly cool down and thus change the reading. The use of various dyes soluble in the melted heat sensitive material, which wick into an absorbent sheet has been notes as one method for solving this problem. Of course, these thermometers can only be used once.
In U.S. Pat. No. 3,704,895 the concept of providing an additional component along with the heat sensitive mixture for the purpose of magnifying the change in state is taught. In one embodiment a higher melting point component which forms an eutectic mixture having a lower melting point than the heat sensitive compound is used. It is noted that said higher melting point component as disclosed does not include a polymer.
U.S. Pat. No. 3,859,856 teaches the use of specific super coolable materials which maintain the melted state for an extended time after the thermometer is removed from the environment, the temperature of which is being measured. The materials taught in this patent are inherently super coolable such as, for example, the eutectic mixture of sodium thiosulfate pentahydrate and sodium acetate trihydrate. In order to reuse the thermometers of this patent, they must be `super cooled` for example, by refrigeration in order to convert the melted materials back to a crystalline state. This refrigeration step is unnecessary with the thermometers of the instant invention. As will be further disclosed below, the instant invention relates to a method of retarding the initiation of crystallization thereby making all heat sensitive materials which undergo reversible visible change by means of crystallization maintain the melted state for a time sufficient to read the temperature. However, unlike the prior art thermometers which utilize `super coolable` materials, a super cooling step, e.g. refrigeration, is not necessary prior to reuse. Thus the flexibility of the instant invention over the teaching of this patent will be apparent.
U.S. Pat. No. 3,956,153 teaches the use of nucleating agents to promote the recrystallization of a heat sensitive material that has undergone phase transition. The instant invention in contradistinction relates to maintaining the state, after phase transition, for a time sufficient to enable the user of the thermometer to read the temperature, however, the use of nucleating agents is not precluded from the scope of the instant invention, since many of the known heat sensitive materials which are useful in this invention tend to super cool. The presence of a nucleating agent prevents this from occurring, while the presence of the polymer extends the time prior to crystallization of the compositions described in U.S. Pat. No. 3,956,153 from a range of 15-30 seconds to a range of from 45 seconds to about 3 minutes or more. In this range it is easier for the user to read the temperature without concern with the time involved in taking the reading.